Note: Descriptions are shown in the official language in which they were submitted.
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The inventlon relates to magneslte-based refractory
material, whlch may be shaped or unshaped. It is understood that
shaped or unshaped refractory compoun~s of the kind named lnclude
both shaped articles, such as refractory brlcks, as well as un-
shaped substances. The shaped articles can be elther ln the drled
state, ln the annealed state, ~,.e. followlng treatment at a tem-
perature between 400 and 800C, or ln a burnt state, i.e. follow-
lng ~irlng usually at temperatures between 1600 and 1800C.
A burnt, refractory moulded body made of magnesla as the
main constituent wlth admixture of a zlrconlum oxlde-rlch materlal
ls dlsclosed ln German Auslegschrlft No. (GE-AS) 22 49 814. The
content of ZrO2 in the moulded body is between 1 and 5% by weight.
Furthermore, ln G~-AS 2Z 49 814, lt ls lndlcated that the zlrconl-
um oxlde-rlch admlxture should have a graln slze of 0.3 to 5 mm,
or the portlon below 1 mm grain slze should not be more than 50%
by welght of the ~lrconlum oxlde quantlty, preferably, however, 0%
b~ welght. The sinterlng propertles are to be lmproved by thls,
- l.e. better compresslon and lower poroslty of the magnesla or the
moulded bodles manufactured therefrom should already be achleved
at low burnlng ternperatures.
Refractory burnt or unburnt brlcks and substances based
on magnesla and contalnlng a ZrO2 admixture of 0.5 to 6% by welght
are dlsclosed ln German Offenlegungsschrift No. (GE-AS) 25 07 556.
However, the brlcks or substances described in GE-OS 25 07 556
contaln not less than 3'~ by welght of an oxlde of R2O3 (wherein R
ls Ee, Al or Cr) and not less than 2% by weight of Fe203, each
based on the quantity of the magnesia constltuents; moreover, they
can contaln admlxtures of
. .
h
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chrome ore or magnesite chrome sinter in quantities up to 50% by
weight, based on the entire mixture. The refractoriness of
sintered magnesia products is to be increased by the admixtures,
i.e. the formation of low-melting reaction products is to be
avoided.
Furthermore, a method for the manufacture of burnt re-
fractory magnesia bricks is disclosed in German Patent No.
26 46 430. In this method, a zirconium compound is added to the
starting mixture so that the ZrO2 content amounts to up to 5.5% by
weight. However, in this previously known method an A12O3
carrier, such as fused corundum or calcined alumina, is added in a
quantity of 2 to 10% by weight so that in this case also, as in
GE-OS 25 07 556, spinel is formed.
In addition, a refractory compound based on magnesia
spinel with a content of 10 to 50% by weight of spinel clinker is
described in German Patent No. 36 14 604. This compound also
contains 0.5 to 4% by weight of zirconium oxide. This should
promote incrustation even at high temperatures, particularly dur-
ing overheating in the sintering zone of rotary kilns for the
cement industry. The incrustation from the clinker melt is
desired in the cement industry in order to protect in this way the
actual refractory bricks from thermochemical attacks by the cement
clinker melt and to improve the thermal insulation to the out-
side.
It is the object of the present invention ts provide
shaped or unshaped refractory compounds based on extremely pure,
low-iron magnesite material which has a high temperature stability
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at temperatures above 1900C, a high stability of volume, i.e. no
shrinkage, good chemical resistance to the charging material dur-
ing operating conditions, high structural flexibility and with
which, for example furnaces, particularly rotary kilns, can be
lined with bricks without joint grouting. High temperatures occur
particularly with use in rotary kilns for manufacturing burnt
dolomite or burnt magnesite, and also in shaft kilns during the
manufacture of burnt lime. The temperatures occurring there are
substantially higher than the temperatures in rotary kilns for
manufacturing cement (approx. 1500C).
It was surprisingly found that refractory compositions
of the kind named have the aforenamed properties when the magnes-
ite starting material used for the manufacture is sintered or
fused magnesite having a purity of at least 97% by weight of MgO
and when the refractory compositions contain as admixture 10 to
25% by weight based on MgO of a zirconium compound having a ZrO2
content of at least 98~ by weight.
With the refractory compositions according to the inven-
tion there is no spinel binding as with refractory compositions or
refractory bricks containing spinels.
In the main burning zones of furnaces for manufacturing
burnt dolomite, magnesite or lime, temperatures above 2000C pre-
vail in the main burning zone of modern installations so that mag-
nesia spinel bricks or magnesia chromite bricks no longer have the
desired durability and cannot be used. For example, magnesia
chromite bricks and also chrome magnesia bricks undergo consider-
able shrinkage of volume; moreover, calcium chromate is formed at
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these high temperatures, which may result in contamination of the
environment. Low-melting calcium aluminate is formed during the
manufacture of burnt dolomite where magnesia spinel bricks are
used.
According to one preferred embodiment, the amount of
zirconium compound in the starting material is 12 to 17% by
weight, based on MgO. Refractory bricks with excellent resistance
to sudden changes of temperature are obtained hereby.
According to a further preferred embodiment, the total
amount of MgO and ZrO2 is at least 97~ by weight of the en tire
composition in the starting material. By using starting materials
which meet these purity requirements, the formation of spinels is
almost entirely avoided so that the occurrence of interfering
reactions is further reduced.
According to a further preferred embodiment baddeleyite
is used as the zirconium compound in the starting materials of the
refractory compositions according to the invention. The baddeley-
ite usually have a particle size of less than 0.5 mm. Refractory
compositions with excellent stability of volume or substances
which are particularly well suited for lining without joint
grouting are obtained herewith.
According to a further preferred embodiment, sulfite
liquor is used as a temporary binding agent. The use of this
binding agent is particularly suitable in the refractory composi-
tions according to the invention since no interfering oxidative
constituents can be introduced through this into the starting
material and thus also not into the shaped or unshaped product
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manufactured from this starting material.
According to a further preferred embodiment, the
sintered or fused magnesite in the starting material has the
following particle size distribution:
3.5 to 6.0 mm 5 to 10%
1.0 to 3.5 mm 25 to 40%
0.09 to 1.0 mm 5 to 20%
~ 0.09 mm 25 to 40%
A particularly compact structure of the products manufactured from
such starting material is achieved -through this, in particular an
excellent stability of volume.
According to a further preferred embodiment, the propor-
tion of refractory material in the starting material having a
grain size of < 0.09 mm amounts to at least 30% by weight, prefer-
ably between 35 and 45~ by weight. A particularly compact and
flexible structure of the finished product is achieved through
this, the chemical resistance to the charging material, i.e. the
dolomite, magnesite or lime to be burnt being improved.
As was already described above, the refractory composi-
tions according to the invention can be used particularly in theform of burnt moulded bodies or refractory bricks to line furnaces
or high temperature zones or main burning zones of furnaces to
manufacture burnt dolomite, burnt lime or burnt magnesite.
The shaped or unshaped refractory compositions according
to the invention are manufactured by simply mixing together the
starting materials, whereby a suitable temporary binding agent,
particularly sulfite liquor, is added. If the product is supplied
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to the consumer in the form of dry matter, a solid temporary
binding agent, for example synthetic resins, polyester resins,
phenolic resins, etc., can be used, possibly also magnesium sul-
fate. If the refractory composition according to the invention is
manufactured in the form of shaped parts, a preferably temporary
binding agent is likewise added to the starting mixture, and also
the required quantity of water. The shaped parts are manufactured
from this material in suitable moulds, preferably by pressing,
particularly isostatic pressing, and following removal from the
mould the shaped parts are dried, usually at temperatures between
100C and 180C, possibly annealed, usually at tempera-tures
between 400C and 800C, and possibly also fired usually at
temperatures between 1600C and 1800C.
The invention will be explained in greater detail on the
basis of the following example.
Example
A starting material was prepared from 85 parts by weight
of sintered magnesia with an MgO content of 97.5~ by weight and 15
parts by weight of baddeleyite with a minimum content of 98.5~ by
weight of ZrO2.
The 85 parts by weight of sintered magnesia had the fol-
lowing particle size distribution:
3.5 to 5.0 mm 10 parts by weight
1.0 to 3.5 mm 35 parts by weight
0.09 to 1.0 mm 10 parts by weight
not more than 0.09 mm 30 parts by weight
The baddeleyite has a particle size of 0 to 0.5 mm.
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Following thorough mixing with a sufficient quantity of
sulfite liquor, this starting material was mixed and subsequently
pressed with a pressing power of 113 to 118 N/mm2 into bricks.
Following removal from the mould, the bricks were dried at 120C
and subsequently subjected to firing between 1740 and 1760C.
The burnt bricks had a bulk density of 3.12 g/cm3, a
porosity of 17%, a cold compressive strength, KDF, of 60 N/mm2 and
a V-module of 6000 N/mm2.
The burnt bricks were subjected to a corrosion test dur-
ing heating in contact with dolomite powder at 2000C, whereby nocorrosion could be ascertained.
Their resistance to sudden changes of temperature, TWB,
had a value of >30. The bricks had excellent deformation proper-
ties so that they could be used in the main burning zone of a
rotary kiln.
